1. Field of the Invention
The present invention relates to an encoder to be mounted in various apparatuses such as an optical apparatus and to be used to detect a movement amount, a position and a velocity of a movable member provided in the apparatus.
2. Description of the Related Art
Such an encoder includes a scale provided with a periodic pattern including multiple gratings periodically arranged, and a sensor that outputs a periodic signal that periodically changes with the periodic pattern in response to a movement relative to the scale. The encoder further includes a signal processor that calculates a relative movement amount (detected value) between the scale and the sensor by using the periodic signal output from the sensor.
In such an encoder, when a positional relation between the sensor and the scale is not in an ideal state, the detected value may contain an error. Japanese Patent Laid-open No. 2012-83153 discloses an encoder including multiple sensors (light-receiving element arrays) and configured to calculate, by weighted averaging of positions respectively detected using outputs of the sensors based on values of the sensor outputs, a position as a detected value with a reduced error.
However, another encoder includes a scale provided with multiple periodic patterns that have mutually different periods and are arranged adjacent to each other. In this encoder, the number of gratings in each periodic pattern and an accumulated change amount of a phase representing a position in a range corresponding to one grating are used to calculate a detected value such as a position. However, the method disclosed by Japanese Patent Laid-open No. 2012-83153 cannot be applied to such an encoder.
This is because a movement amount is not the detected value calculated from the number of gratings and the accumulated change amount of the phase, but is calculated from the detected value and a grating pitch. In other words, even if the same detected values are obtained, different grating pitches provides different movement amounts, and thereby the same detected values do not correspond to a same position. For example, a position moved by one period from a reference position for a sensor reading a periodic pattern whose period is 99 μm is different from a position moved by one period from the reference position for a sensor reading a periodic pattern whose period is 100 μm.